An interleukin-1 (IL-1) lymphocyte-activating factor, with an approximate molecular weight of 13,000–16,000 was isolated from the culture supernatants of a murine macrophage-like cell line, P388D1. Contrary to the general belief that murine mediator is incapable of stimulating human lymphocyte mitogenic responses, leukoagglutinin (LA)-induced mitogenesis of peripheral blood lymphocyte (PBL) were significantly affected by murine IL-1. On one hand, when PBL were obtained from either normal healthy individuals or from cancer patients who still possessed normal levels of general immunocompetence, their mitogenic responses were not augmented any further by the murine mediator. Instead, slight but significant suppression were noted in most cases. On the other hand, the LA-induced responses of PBL from 5 immunodepressed cancer patients were markedly augmented by murine IL-1.